The present invention relates generally to laser shock peening and, more particularly, to a method and system for on the fly analog switching on the fly of laser energy levels during the laser shock peening process.
Laser shock peening or laser shock processing as it is also referred to, is a process for producing a region of deep compressive residual stresses in a surface area of an article. Laser shock peening typically uses one or more radiation pulses from high energy, i.e., 1 to 50 joules of energy or more, pulsed laser beams to produce an intense shockwave at the surface of an article. Exemplary methods of LSP are disclosed in U.S. Pat. No. 3,850,698 entitled “Altering Material Properties”; U.S. Pat. No. 4,401,477 entitled “Laser Shock Processing”; and U.S. Pat. No. 5,131,957 entitled “Material Properties.” Further, the use of lower energy laser beams in LSP is disclosed in U.S. Pat. No. 5,932,120 entitled “Laser Shock Peening Using Low Energy Laser.” Laser shock peening, as understood in the art and as used herein, means utilizing a pulsed laser beam from a laser beam source to produce a strong localized compressive force on a portion of a surface by producing an explosive force at the impingement point of the laser beam by an instantaneous ablation or vaporization of a thin layer of that surface or of a coating (such as tape or paint) on that surface which forms a plasma.
Laser shock peening is being developed for many applications in the gas turbine engine field, some of which are disclosed in the following: U.S. Pat. No. 5,756,965 entitled “On The Fly Laser Shock Peening”; U.S. Pat. No. 5,591,009 entitled “Laser Shock Peened Gas Turbine Engine Fan Blade Edges”; U.S. Pat. No. 5,531,570 entitled “Distortion Control For Laser Shock Peened Gas Turbine Engine Compressor Blade Edges”; U.S. Pat. No. 5,492,447 entitled “Laser Shock Peened Rotor Components For Turbomachinery”; U.S. Pat. No. 5,674,329 entitled “Adhesive Tape Covered Laser Shock Peening”; and U.S. Pat. No. 5,674,328 entitled “Dry Tape Covered Laser Shock Peening.”
Laser shock peening has been utilized to create a compressively stressed protective layer at the outer surface of an article which is known to considerably increase the resistance of the article to fatigue failure as disclosed in U.S. Pat. No. 4,937,421 entitled “Laser Peening System and Method”. These methods typically employ a curtain of water flowed over the article or some other method to provide a plasma confining medium. This medium enables the plasma to rapidly achieve shockwave pressures that produce the plastic deformation and associated residual stress patterns that constitute the laser shock peening effect. The curtain of water provides a confining medium, to confine and redirect the process generated shockwaves into the bulk of the material of a component being laser shock peened, to create the beneficial compressive residual stresses.
The pressure pulse from the rapidly expanding plasma imparts a traveling shockwave into the component. This compressive shockwave initiated by the laser pulse results in deep plastic compressive strains in the component. These plastic strains produce residual stresses consistent with the dynamic modulus of the material. The many useful benefits of laser shock peened residual comprehensive stresses in engineered components have been well documented and patented, including the improvement on fatigue capability. These comprehensive residual stresses are balanced by the residual tensile stresses in the component. The added residual tensile stresses may locally lower the fatigue capability of components and thus, should be reduced and/or minimized.
Laser shock peening is performed at selective locations on the component to solve a specific problem. The balancing tensile stresses usually occur at the edge of the laser shock peened area. Small narrow bands or lines of tensile stresses can build up immediately next to the laser shock peened patch area along the edges of the patch. Extensive finite element analyses are done to determine where these tensile stresses will reside and the LSP patches are designed and dimensioned such that the tensile band(s) end up in an inert portion of the article or component (i.e., not at a high stress line in one of the flex, twist or other vibratory modes). It is desirable to reduce the level of these tensile stresses in the transitions area between the laser shock peened and non-laser shock peened areas.
During the laser peening process, the laser head moves in linear movements across the treated area. The time delay between each linear space is approximately one second. Most applications using laser shock peening require different energy levels and laser heads for select areas that are being laser shock peened. One energy level will be used for several linear areas, and then a switch must be made to another energy level or another laser head for the next linear area. Since the delay time between linear areas is approximately one second, the switch between laser heads or energy levels should be made within that one-second delay to ensure optimal energy use and the most efficient system. In the past, digital switches were used to facilitate the switching, but the timing for these digital switches were greater than one second, resulting in an energy and time loss during the switching delay for the energy level and laser head switching.
Therefore, there is a need for a switching mechanism that can provide a switching time of less than one second so the changing of energy levels and laser heads can be accomplished during the delay between linear areas.